Fix compilation of Python bindings

Summary:
We only need to pass TRANSLATION_DOMAIN when compiling the C++ code since it's the only part that contains i18n() calls.

Reason: The klocalizedstring code fails to compile if that is defined (see how klocalizedstring.cpp undefines it)

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5981